As is well known, a stairlift assembly broadly comprises a rail which is mounted on a stairway, a carriage mounted on the rail for movement there along, and a chair mounted on the carriage. In the case of straight line stairlifts, where the stairlift assembly only traverses a straight section of stair, the rail is mounted at a constant angle to the horizontal and, accordingly, the chair can be fixed with respect to the carriage. In the case of curved stairlifts, however, sections of the rail are aligned at different angles with respect to the horizontal. Thus, a facility has to be included to rotate the chair with respect to the horizontal, to ensure the seating surface of the chair remains horizontal at all positions of the carriage along the rail.
Traditionally, the leveling function of a curved stairlift has been fabricated, at least in part, into the rail and the provision of the leveling function contributes significantly to the overall cost of the rail.
One common, early, example of a rail for a curved stairlift is formed from a metal section of constant cross-section. This form of rail operates in conjunction with a carriage on which the chair is rotatably mounted. A leveling bar is fixed, at varying angles, to the outer surface of the rail to provide a surface against which a linkage, forming part of the carriage, can bear. As the carriage moves along the rail, the action of the linkage against the leveling bar causes the chair to rotate with respect to the carriage, and thus maintain the seating surface level.
This form of leveling has been widely adopted but has its drawbacks. Firstly, considerable care must be taken, when fabricating the rail, to position and fix the leveling bar accurately. Secondly, there are limits to which one can reduce the size of the rail in order to accommodate the variations in alignment required by the leveling bar. Further, the need to add a leveling bar to the exterior surface of the rail inevitably detracts from the aesthetics of a stairlift installation.
As an alternative to the leveling arrangement described above, curved rails have been formed from two standard section tubes positioned in common vertical planes with the carriage being in rolling contact with both the upper and the lower tubes. By varying the distance between the tubes (which are typically of considerably smaller section than the constant metal sections referred to above), the carriage is caused to rotate with respect to the rail. Since, in this arrangement, the chair is fixed to the carriage, the chair will rotate with the carriage. Thus, by arranging variations in the tube spacing to coincide with changes in rail angle, the chair can be maintained level. One example of this form of rail is described in International Patent Application WO 96/20125.
In a variation to the twin tube arrangement, the rail is formed as an I-beam with the distance between the cross pieces of the I being varied to effect carriage rotation in the manner described above.
Again the twin-tube or I-beam rails have their drawbacks. Firstly, while the individual tubes may be quite small in section, the composite of the two spaced tubes (or varying section I-beam) results in a rail having significant depth. Further, it is difficult and costly to accurately locate and fix the components forming the rail so as to ensure accurate leveling—both in the direction of travel of the carriage, and perpendicular thereto.
Given the above drawbacks, considerable effort has been expended in arriving at solutions which transfer the leveling function from the rail to the carriage. The most successful of these solutions involve the use of a separate, electronically controlled, chair leveling motor to rotate the chair with respect to the carriage at each point along the rail at which the angle of the rail changes with respect to the horizontal. For convenience this mode of leveling will be referred to herein as electronic leveling.
One commonly available, electronically-leveled, stairlift incorporates a rail formed from a single round tube, the tube having a stability bar fixed to, and extending along, the outer surface thereof. The stability bar provides a surface against which reaction rollers can bear, to prevent the carriage from rotating about the rail.
An example of such a form of rail is disclosed in International Patent Application WO 97/12830. In this particular example the stability bar projects vertically down from the lower edge of the tubular rail and incorporates the rack which forms part of the drive system for the carriage along the rail. The advantage of this type of rail is that its basis is readily available, standard section, round metal tube. Such tube can be bent using readily available bending equipment which is an important consideration since all curved rails require a bending operation. Generally speaking, forming curves in non-round sections leads to unacceptable distortion of the section. As a consequence, bends have to be specially fabricated.
A significant disadvantage of the single round tube rail is that additional manufacturing input is required to form the stability bar and to fix the stability bar to the rail. Forming the stability bar also involves considerable material wastage.
The single round tube arrangement typically has less overall bulk than the spaced twin tube arrangement described above. However, the diameter of the tube currently used, being in the order of 76 mm, needs to be quite significant in order to give the requisite bending strength to the rail. This, in turn, severely limits the radii through which the tube can be bent. As a result, when the rail is to follow, say, a right-angled bend in a stairway, the rail will protrude from a corner a significant distance into the stairway. Further, the rail cannot be formed into a tight inside bend at the top or bottom of the stairs to enable the carriage to be moved into a convenient storage position, off the stairway.
In our published International Patent Application WO 02/064481 we describe a rail formed from two standard section metal tubes located one on top of the other, the individual tubes being considerably smaller in diameter than that from which the single tube rails described above, are formed. In this arrangement each tube is bent individually, using standard pipe bending equipment, to a desired shape. The two tubes are then nested together and fixed.
This arrangement has the advantage over the single round tube rail that, because the individual tubes are much smaller in diameter than the single tube, much smaller inside bends can be formed in the rail. Further, because the overall rail is non-circular in section, its inherent form provides resistance to rotation of the carriage about the rail and does not, therefore, require the addition of a stability bar. However, in experimental work conducted to date, we have not been able to successfully resolve the problems associated with forming rails from two individual tubes. The individual tubes must each be bent with slightly different curvature so that the two fit together accurately. Further, particularly on inside/outside bends and helicals, any slight vertical misalignment between the two tubes can lead to unacceptable forward or rearward tilting of the carriage.
It is an object of the present invention to provide a stairlift rail which goes some way in meeting the drawbacks of existing curved rail arrangements as outlined above, or which will at least provide a novel and useful alternative.